I don't have much to say. I am a 20 yr guy, and have struggled with P & M since i was 14. Longest ever streak i can remember was around 35-40 days. I have done countless hachlatas, all to no avail. Just sucks always feeling defeated, and I truly can't grasp that it's possible for me to win this problem, and that it isn't too late and the damage can still be reversed. I am very scared that when I get married it will chas veshalom have terrible irreversible consequences and Im too far gone. I obviously don't know that and I am speculating, but the fear is real. And the Shame.

welcome aboard! You're in the right place with many people here having similar stories as yours and B"H finding heeling. Look around at all the threads, and keep posting!

Welcome my friend!!
Kudos for your first post - it takes great courage to open up!

May i ask what techniques have you tried so far? How did you make it to 40 days clean?

Stick around, keep posting and iyH soon you'll be able to see some light!
Here at GYE countless struggling Yidden have proven that, yes, it's possible to brake free and to do real Teshuva (thus reversing damage).
Re your fear of marriage, there is really nothing to worry about. The images you've seen will fade away. If anything, once you get this beast under control your marriage will only have gained from your journey as you will not confuse lust and love.

A great way to start is by reading "The battle of The Generation" (link in my signature). See also this thread. 

Wishing you tremendous Hatzlacha!

Keep strong,
UpAndDown

Welcome! 
You're in the right place! Let us know if there is anything we can do for you. Also, take full advantage of all the resources here. They really help!! 

Well, this time is different. You're part of the GYE family now!!

EEverything is reversible, have you heard of Teshuva?

A fellow struggling bacher,
   Yosef

Welcome!
It's amazing that you have the courage to take this on. I wanted to add that you are much more likely to succeed if you don't just seek to avoid negative for 90 days, but rather also do something positive to strengthen yourself each day for 90 days in a row.
Some good ideas are below in my signature.

Welcome to the warmest family in the world!

Here you will find true care, concern, and warmth.
Here you will learn that you CAN break free!
As you may know from your time here, there are many tools here to help you in this fight, some of them you may not be familiar with.
There is the F2F Program, the Vaad Program* (click here for an explanation of what the vaad is), and the book The Battle of the Generation - many have found this very helpful in reframin' the struggle.
Posting is a great way to connect, learn, and grow also. 
(The Hall of Fame Thread is an awesome compilation of some the great threads on GYE.)

There is also an extremely powerful tool of accountability, friends, and mentors that has helped hundreds - myself included.
HHM - Hashem Help Me - is the mentor-in-chief around here. He's reachable at michelgelner@gmail.com.
Some of the other great guys here are Eerie - 1gimpelovitz@gmail.com, Muttel - muttel15@gmail.com,   Reb Akiva - mevakesh247@gmail.com iwantlife - iwantlifegye@proton.me minhamayim - minhamayim1@gmail.com amevakesh - amevakesh23@gmail.com iwannalivereal - iwannalivereal@gmail.comcleanmendy - mendelclean1234@gmail.com

Keep postin', you'll see, the oilam is here for you.

Lookin' forward to seeing great things from you!
And don't forget, as always, KOMT!!

Thanks for the encouragement I appreciate it. I made 40 days clean when I did a Mivtza to go a month clean (Hachlatah with reward from someone sponsoring). I'll keep posted with progress